1. Field of the Invention
The present invention relates to an organic light emitting diode (OLED) display, in particular, to an organic light emitting diode (OLED) display capable of increasing the life thereof and of improving display quality and a method of driving the same.
2. Discussion of the Related Art
Recently, various flat panel displays (FPD) having reduced reducing weight and volume compared to cathode ray tubes (CRT) based displays are being developed. The FPDs include liquid crystal displays (LCD), field emission displays (FED), plasma display panels (PDP), and electroluminescence devices.
Because the structure and manufacturing processes of the PDP are simple, the PDP is spotlighted as a display that is light, thin, short, and small and that is advantageous for use in large screen display applications. However, the PDP has low light emitting efficiency and brightness and large power consumption. A thin film transistor (TFT) LCD in which a TFT is used as a switching device is one of the most widely used FPDs. However, because the TFT LCD is a non-emitting device, the TFT LCD has a narrow viewing angle and low response speed.
Electric field light emitting devices are classified as inorganic light emitting diode displays or organic light emitting diode (OLED) displays in accordance with the material of an emission layer. In particular, the OLED display responds at high speed and has high light emitting efficiency, brightness, and a wide viewing angle due to using a self-emitting device.
An OLED for a display is illustrated in FIG. 1. The OLED include organic compound layers such as a hole injection layer (HIL), a hole transport layer (HTL), an emission layer (EML), an electron transport layer (ETL), and an electron injection layer (EIL) formed between an anode electrode and a cathode electrode.
The organic compound layers include a hole injection layer (HIL) 78e, a hole transport layer (HTL) 78d, an emission layer (EML) 78c, an electron transport layer (ETL) 78b, and an electron injection layer (EIL) 78a. 
When a driving voltage is applied between the anode electrode and the cathode electrode, holes that pass through the HTL 78d and electrons that pass through the ETL 78b, meet in the emission layer (EML) 78c to form excitons. As a result, the EML 78c generates visible rays.
The OLED display includes a matrix of pixels each including an OLED, and controls the brightness of pixels selected by a scan pulse in accordance with the gray scale of digital video data.
The OLED displays may be operated using a passive matrix method or using an active matrix method in which the TFT is used as the switching device.
In the active matrix method, a TFT employed as an active device is selectively turned on to select pixels and the luminescence of the pixels is sustained by a voltage sustained in a storage capacitor.
FIG. 2 is a circuit diagram schematically illustrating one pixel in the OLED display employing the active matrix method.
Referring to FIG. 2, the pixel of the OLED display in the active matrix method includes an OLED, a data line DL and a gate line GL that cross each other, a switch TFT SW, a driving TFT DR, and a storage capacitor Cst. The switch TFT SW and the driving TFT DR are realized using N-type MOS-FETs.
The switch TFT SW is turned on in response to the scan pulse from the gate line GL to allow a current flow through a current path between the source electrode and the drain electrode thereof. In the on time period of the switch TFT SW, the data voltage from the data line DL is applied to the gate electrode of the driving TFT DR and the storage capacitor Cst via the source electrode and the drain electrode of the switch TFT SW.
The driving TFT DR controls the current that flows through the OLED in accordance with a voltage difference Vgs between the gate electrode and the source electrode thereof.
The storage capacitor Cst stores the data voltage applied to one electrode thereof to maintain the voltage supplied to the gate electrode of the driving TFT DR for one frame.
The OLED has the structure illustrated in FIG. 1. The OLED is connected between the source electrode of the driving TFT DR and a low potential driving voltage source VSS.
The brightness of the pixel illustrated in FIG. 2 is in proportion to the current that flows through the OLED. The current flowing through the OLED is determined by the difference voltage between the gate voltage and the source voltage of the driving TFT DR, the threshold voltage of the driving TFT DR, and the data voltage as illustrated in EQUATION 1.
                              Vgs          =                      Vg            -            Vs                          ⁢                                  ⁢                              Vg            =            Vdata                    ,                      Vs            =            Vss                          ⁢                                  ⁢                                                            Ioled                =                                ⁢                                                      k                    2                                    ⁢                                                            (                                              Vgs                        -                        Vth                                            )                                        2                                                                                                                          =                                ⁢                                                      k                    2                                    ⁢                                                            (                                              Vdata                        -                        Vss                        -                        Vth                                            )                                        2                                                                                                          [                  EQUATION          ⁢                                          ⁢          1                ]            
In EQUATION 1, Vgs represents a difference voltage between the gate voltage Vg and the source voltage Vs of the driving TFT DR, Vdata represents the data voltage, Vss represents the low potential driving voltage, Ioled represents the driving current, Vth represents the threshold voltage of the driving TFT DR, and k represents a constant value determined by the mobility and the parasitic capacitance of the driving TFT DR
As illustrated in EQUATION 1, the current Ioled of the OLED is remarkably affected by the threshold voltage Vth of the driving TFT DR.
In general, when a gate voltage of the same polarity is applied to the gate electrode of the driving TFT DR for a long time, gate-bias stress is increased so that the threshold voltage Vth of the driving TFT DR is increased, thus changing the operation characteristic of the driving TFT DR. A change in the operation characteristic of the driving TFT DR is illustrated in the experiment result of FIG. 3.
FIG. 3 illustrates that the characteristic curve for a sample hydrogenised amorphous silicon (A-Si:H) TFT changes when positive gate-bias stress is applied to an A-Si:H TFT whose channel width/channel length W/L is 120 μm/6 μm. In FIG. 3, the axis of abscissa represents the gate voltage V of the A-Si:H TFT as the sample and the axis of ordinate represents current A between the source electrode and the drain electrode of the A-Si:H TFT as the sample.
FIG. 3 illustrates the shift in the threshold voltage and the transmission characteristic curve of the TFT with the increase in voltage application time when a voltage of +30V is applied to the gate electrode of the A-Si:H TFT as an example. As illustrated in FIG. 3, as the duration for applying a positive voltage to the gate electrode of the A-Si:H TFT increases, the transmission characteristic curve of the TFT moves to the right and the threshold voltage of the A-Si:H TFT increases. In the illustrated example, the threshold voltage increases from Vth1 to Vth4.
The increase in the threshold voltage of the driving TFT DR makes the operation point of the driving TFT DR unstable, reducing the life of the display. For example, in the pixel circuit illustrated in FIG. 2, when the threshold voltage Vth of the driving TFT DR increases from 1.5V to 2V, although the same data voltage is applied, the amount of driving current is reduced to 70% of the initial value. In addition, when pixels are driven by data voltages having different magnitudes in a uniform period, the deterioration degree of the driving TFT DR of the pixel to which a relatively large data voltage is accumulatively applied is larger than the deterioration degree of the driving TFT DR of the pixel to which a relatively small data voltage is accumulatively applied. Therefore, when the same data voltage is subsequently applied to the pixels, the amount of the current that flows through the OLED varies with each pixel, reducing display quality.